This invention generally relates to control systems for display devices. More particularly, this invention relates to brightness control systems for backlight display devices.
Backlight display devices are used in a variety of consumer and industrial products to display data, charts, graphs, messages, other images, information, and the like. Backlight display devices, which may be backlit or frontlit, have a backlight positioned to provide light for a display panel. The backlight may be a fluorescent tube, an electroluminescent device, a gaseous discharge lamp, a plasma panel, and the like. The display panel display may be a passive or active matrix liquid crystal display (LCD). The backlight and display panel are connected to control circuitry, which is connected to a voltage supply. Alternatively, the display may be emissive such as an organic LED display which does not have a backlight. The display device may be separate or incorporated with other components, such as a dashboard in an automobile or other vehicle, a portable electronic device, and the like.
In general, the luminance of the backlight is adjusted to provide the desired brightness of the backlight display device. A driver circuit controls the backlight luminance by increasing or decreasing the drive current to the backlight. The drive current typically is adjusted in relation to the environment and user preferences. A poorly-lit environment usually requires less brightness, and thus a lower drive current, than a brightly-lit environment. Different users may have different desired brightness levels. The brightness may be changed automatically or manually. The backlight display device may have a switch, a keypad, a touch screen, a remote device, or the like to adjust the brightness. The backlight display device may have an automatic brightness control system, which also may include manual adjustments to brightness.
The backlight luminance is proportionate to the drive current. However, the efficiency of the backlight may change during operation of the backlight display device. The changing efficiency varies the backlight luminance and hence the brightness of the backlight display device. The efficiency of the backlight display device usually is low at start-up and then increases during a xe2x80x9cwarm-upxe2x80x9d period. The efficiency may or may not be proportional to temperature. The efficiency during start-up may be very low, thus providing a dim or no image until the warm-up period is completed. To reduce the warm-up period, some backlights have resistive wire wrapped around the backlight or other backlight heater techniques. Some backlights are designed to quickly self-heat. Even after the warm-up period, the efficiency of the backlight may change during operation of the backlight display device, such as when the backlight display device moves through colder and warmer ambient conditions. The backlight efficiency may change due to the drive current level itself. Higher drive currents tend to increase the backlight temperature and lower drive currents tend to decrease the backlight temperature, thus changing the efficiency. The backlight efficiency also may change for other reasons such as little or no lumen maintenance over time and variations in thermal resistance and circuit operation. Unless the drive current is adjusted for changes in the efficiency of the backlight, the brightness of the backlight display device may vary during operation. A user also may adjust or readjust the brightness in response to the changing brightness.
Many backlight display devices adjust the drive current level based on the temperature of the backlight. A temperature sensor senses the temperature of the backlight. The temperature is compared to a look-up table of drive current levels for the backlight display device. The look-up table provides drive current levels (Vo) as a function of temperature (T) for each brightness level in the dynamic range. For each brightness level, an entire series of Vo vs. T data values is required. Based on the temperature and brightness level, a drive current level is selected for the backlight display device. This approach may lead to very large look-up tables. Each brightness level needs a large number of Vo vs. T data values to properly control the luminance of the backlight. Additional Vo vs. T data values may be necessary to give a more gradual appearance to changes in the drive current level at a constant desired brightness. Other factors may increase the number of brightness levels.
More brightness levels may be needed in applications having wider dynamic ranges and variable resolution of the brightness. A wider dynamic range generally needs more brightness levels than a narrower dynamic rage. The dynamic range corresponds to the use of the display device. A narrow dynamic range may cover one or a small number of uses such as daylight use, nighttime use, or the like. A wide dynamic range may cover several uses such as daylight use, nighttime use, dusk-to-dawn use, and the like.
The number of brightness levels also increases when the desired brightness resolution increases. More brightness levels provide higher resolution than fewer brightness levels with sufficient resolution. Brightness adjustments generally need to have variable resolution because of how the human eye perceives changes in brightness. The human system perceives changes in brightness non-linearly and logarithmically. A user perceives a brightness change from about 10 nits to about 12 nits as essentially equal to a brightness change from about 100 nits to about 120 nits. As the brightness level decreases, more brightness resolution provides brightness step changes perceived as uniform by a user. Thus, a backlight needs more brightness resolution at lower luminance levels and less brightness resolution at higher luminance levels.
The look-up table usually is provided in a memory or other data storage device in the backlight display device. As more brightness levels are required, a larger memory device is needed. The look-up tables also may be more difficult to implement in a brightness control system and may delay changes to the luminance level backlight device. This approach may be essentially unworkable or expensive for daytime automatic brightness functions that may require greater than about 160 brightness levels to control the display brightness as a function of ambient light.
This invention provides a brightness control system for a backlight display device that uses the efficiency of the backlight to calculate and control the backlight to a desired brightness or luminance for the backlight display device. The backlight efficiency is a function of the backlight temperature. At each backlight temperature, the brightness is linearly proportional to the drive current for the backlight. By using the backlight temperature and backlight efficiency to control the backlight to the desired brightness, the brightness control system may maintain the desired brightness throughout the dynamic range of the backlight display device.
In one aspect, the backlight display device has a display panel, a backlight, a temperature measurement device, and control circuitry. The backlight is positioned next to the display panel. The temperature measurement device is operatively positioned near the backlight. The control circuitry is connected to receive a temperature signal from the temperature measurement device. The control circuitry is connected to provide a drive current to the backlight. The control circuitry determines a backlight efficiency in response to the temperature signal. The control circuitry determines the drive current in response to the backlight efficiency and a desired brightness signal.
In a method for controlling the brightness of a backlight display device, a desired brightness signal is provided for a backlight. A temperature signal is generated that is indicative of the temperature of the backlight. A backlight efficiency is determined in response to the temperature signal. A drive current is provided to the backlight in response to the backlight efficiency and the desired brightness signal.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.